1. Field of the Invention
This invention relates to an air-fuel ratio control system for an internal combustion engine having a plurality of cylinder groups, which carries out air-fuel ratio control per cylinder group, e.g. for each bank in the case of a V-type engine, and more particularly, to an air-fuel ratio control system of this kind, which is capable of detecting deterioration of catalytic converters arranged in the exhaust system.
2. Prior Art
There is conventionally known an air-fuel ratio control system for internal combustion engines, which has oxygen concentration sensors (hereinafter referred to as "the O2 sensors") arranged, respectively, upstream and downstream of a three-way catalyst as a catalytic converter arranged in the exhaust system of the engine, and carries out air-fuel ratio feedback control by calculating a feedback control amount (air-fuel ratio control amount) for controlling the air-fuel ratio of an air-fuel mixture supplied to the engine (hereinafter referred to as "the supplied air-fuel ratio"), based on outputs from the upstream and downstream O2 sensors.
A system of this kind carries out air-fuel ratio control by correcting the air-fuel ratio of a mixture supplied to each cylinder to the same value. To detect deterioration of a three-way catalyst used in an internal combustion engine employing the above air-fuel ratio control method, a method of detecting a deterioration degree of the catalyst has been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 63-97852, which comprises calculating the number of times of inversion of outputs from the downstream O2 sensor for a predetermined time period when the engine is in a predetermined operating condition, and determining that the catalyst is deteriorated when the calculated number of times of inversion exceeds a predetermined value.
In addition to the above-mentioned air-fuel ratio control method, there is also known another airfuel ratio control method, as proposed by U.S. Pat. No. 5,228,287, in which air-fuel ratio feedback control is carried out based on air-fuel ratio control amounts which are different between the cylinder groups. This air-fuel ratio control method which controls per cylinder group is superior in air-fuel ratio control accuracy to the above-mentioned method, and includes various types, such as a method which controls per bank (right bank and left bank) for a V-type engine, and a method which controls per cylinder group (a first group consisting of #1 and #4 cylinders and a second group consisting of #2 and #3 cylinders) for a straight-type four-cylinder engine. In these engines, upstream O2 sensors are provided for respective banks or cylinder groups, a single catalytic converter is arranged, for example, at a location of a converged portion of an exhaust manifold extending from the cylinder groups, and a single downstream O2 sensor is arranged downstream of the catalytic converter.
However, if the above proposed catalyst deterioration-detecting method is applied together with the air-fuel ratio control method for such an engine employing a single catalytic converter, the above feedback control operations are carried out based on different air-fuel ratio control amounts responsive to the outputs from the upstream O2 sensors of the respective cylinder groups, and accordingly exhaust gases emitted from the cylinder groups flow into the single catalytic converter. As a result, it is impossible to accurately measure the O2 storage amount of the catalyst (a time period from inflow of a rich exhaust gas to generation of a rich output from the downstream O2 sensor, or a time period from inflow of a lean exhaust gas to generation of a lean output from the downstream O2 sensor).
More specifically, the air-fuel ratio control amounts for the cylinder groups are independently calculated, so that when a mixture having a lean air-fuel ratio is supplied to one cylinder group, while at the same time a mixture having a rich air-fuel ratio is supplied to the other cylinder group, it becomes unclear whether the total air-fuel ratio is in the rich state or in the lean state. Therefore, it cannot be judged whether O2 molecules are being stored into the catalytic converter or being discharged therefrom. As a result, the O2 storage amount cannot be properly measured, whereby deterioration of the catalyst cannot be accurately detected.
One way to solve the above problem would be to provide a catalytic converter for each of the cylinder groups.
However, even if the catalytic converters are provided for the respective cylinder groups, a single downstream O2 sensor, which will be provided downstream of the catalytic converters, will detect the air-fuel ratio of mixed exhaust gases after purification by the catalysts of the respective cylinder groups. Therefore, by the use of the catalyst deterioration-detecting method as proposed by Japanese Provisional Patent Publication No. 63-97852, referred to above, which makes it a prerequisite that the air-fuel ratio control should be carried out by supplying the same air-fuel ratio control amount to all the cylinder groups, it is impossible to separately detect the O2 storage amount of each of the catalytic converters, leading to incapability of detecting deterioration of each catalyst. To eliminate this inconvenience, a catalytic converter and a downstream O2 sensor may be provided for each cylinder group to detect deterioration of each catalyst, which, however, results in a high cost due to the use of two downstream O2 sensors.